linux-kernel - Re: [PATCH 1/2] topology: use bin_attribute to avoid buff overflow

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Date:   Wed, 2 Jun 2021 11:48:22 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     "tiantao (H)" <tiantao6@...wei.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Tian Tao <tiantao6@...ilicon.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Jonathan Cameron <jonathan.cameron@...wei.com>
Subject: Re: [PATCH 1/2] topology: use bin_attribute to avoid buff overflow

On Wed, Jun 2, 2021 at 9:45 AM tiantao (H) <tiantao6@...wei.com> wrote:
> 在 2021/6/2 14:18, Greg KH 写道:
> > On Wed, Jun 02, 2021 at 02:14:49PM +0800, tiantao (H) wrote:
> >> 在 2021/6/1 12:58, Greg KH 写道:
> >>> On Tue, Jun 01, 2021 at 10:56:49AM +0800, Tian Tao wrote:

...

> >>>>    /**
> >>>> + * bitmap_print_to_buf - convert bitmap to list or hex format ASCII string
> >>>> + * @list: indicates whether the bitmap must be list
> >>>> + * @buf: page aligned buffer into which string is placed
> >>>> + * @maskp: pointer to bitmap to convert
> >>>> + * @nmaskbits: size of bitmap, in bits
> >>>> + * @off: offset in buf
> >>>> + * @count: count that already output
> >>>> + *
> >>>> + * the role of bitmap_print_to_buf and bitmap_print_to_pagebuf is
> >>>> + * the same, the difference is that the second parameter of
> >>>> + * bitmap_print_to_buf can be more than one pagesize.
> >>>> + */
> >>>> +int bitmap_print_to_buf(bool list, char *buf, const unsigned long *maskp,
> >>>> +                  int nmaskbits, loff_t off, size_t count)
> >>>> +{
> >>>> +  int len, size;
> >>>> +  void *data;
> >>>> +  char *fmt = list ? "%*pbl\n" : "%*pb\n";
> >>>> +
> >>>> +  len = snprintf(NULL, 0, fmt, nmaskbits, maskp);
> >>>> +
> >>>> +  data = kvmalloc(len+1, GFP_KERNEL);
> >>>> +  if (!data)
> >>>> +          return -ENOMEM;
> >>>> +
> >>>> +  size = scnprintf(data, len+1, fmt, nmaskbits, maskp);
> >>>> +  size = memory_read_from_buffer(buf, count, &off, data, size);
> >>>> +  kvfree(data);
> >>>> +
> >>>> +  return size;
> >>> Why is this so different from bitmap_print_to_pagebuf()?  Can't you just
> >>> use this function as the "real" function and then change
> >>> bitmap_print_to_pagebuf() to call it with a size of PAGE_SIZE?
> >> Do you mean do following change, is that correct? :-)
> > Maybe, it is whitespace corrupted, and it still feels like this function
> > is much bigger than it needs to be given the function it is replacing is
> > only a simple sprintf() call.
> >
> >> +int bitmap_print_to_buf(bool list, char *buf, const unsigned long *maskp,
> >> +                       int nmaskbits, loff_t off, size_t count)
> >> +{
> >> +       int len, size;
> >> +       void *data;
> >> +       const char *fmt = list ? "%*pbl\n" : "%*pb\n";
> >> +
> >> +       if (off == LLONG_MAX && count == PAGE_SIZE - offset_in_page(buf))
> >> +               return scnprintf(buf, count, fmt, nmaskbits, maskp);
> >> +
> >> +       len = snprintf(NULL, 0, fmt, nmaskbits, maskp);
> >> +
> >> +       data = kvmalloc(len+1, GFP_KERNEL);
> > Why do you need to allocate more memory?  And why kvmalloc()?
>
> Because the memory here will exceed a pagesize and we don't know the
> exact size, we have to call
>
> snprintf first to get the actual size. kvmalloc() is used because when
> physical memory is tight, kmalloc
>
> may fail, but vmalloc will succeed. It is not so bad that the memory is
> not requested here.

To me it sounds like the function is overengineered / lacks thought
through / optimization.
Can you provide a few examples that require the above algorithm?

> >> +       if (!data)
> >> +               return -ENOMEM;
> >> +
> >> +       size = scnprintf(data, len+1, fmt, nmaskbits, maskp);
> >> +
> >> +       size = memory_read_from_buffer(buf, count, &off, data, size);
> >> +       kvfree(data);
> >> +
> >> +       return size;
> >> +}


--
With Best Regards,
Andy Shevchenko